The present invention is related to transistorized amplification stages and more particularly to polarization circuits thereof.
The amplification stages used, for example, in television emitters or transposers consume a certain amount of energy, a certain proportion of such energy being consumed by the polarization circuit. The aim is to reduce the energy consumption of such equipment, particularly in two important cases, to wit:
on the one hand, when the equipment is low-power equipment and has low energy consumption while the cost of energy is very high, for example at isolated locations where the energy is provided by solar cells, windmill generators or the like, a reduction in consumed energy results substantially in a reduction of the operating cost; and PA1 on the other hand when the equipment is high-power equipment and has high energy consumption, a reduction in energy results in a reduction of dissipated thermal energy, which entails mainly a reduction in the manufacturing and installation cost (ventilation and air conditioning).
With this purpose in view it is, first of all, possible to improve the yield of the feeding means, but it is not possible to go very far in this direction. When yield rates near the possible limit-values are reached, i.e. between 80 and 90% each improvement increment of such yield rate leads to increase considerably the manufacturing cost of the feeding means.
With a view to decrease the energy consumption of the equipment the invention proposes to improve the yield rate of the power amplifiers, since they constitute the part of the equipment which consumes the largest proportion of the energy fed thereto. Indeed in the power stages each amplifier is constituted by one or more HF transistors, for example by 64 transistors for producing 1 kilowatt in VHF. The yield of the amplifier is a function--amongst other parameters--of the selected amplification class. The requirements as to the quality of the amplified signal make it necessary to use amplification classes A or AB so as to avoid distortions. These amplification classes generally bring about a poor yield rate. Generally the class A amplifiers are constant collector current polarized amplifiers; for this purpose an image voltage of the collector current in the power transistor is compared in a comparator to a reference voltage obtained by means of a supply voltage divider. This comparator provides a base current that is controlled so that a constant collector current is obtained with a voltage equal to the supply voltage. The power yield rate of such a polarization stage is very low.